Vehicle manufacturers are developing hybrid vehicles to meet the demand for more fuel efficient vehicles. One configuration for a hybrid vehicle may be referred to as a Modular Hybrid Transmission (MHT) vehicle design. In a MHT vehicle, an electric machine is sandwiched between conventional automatic step ratio transmission and the engine. The electric machine is attached to the transmission impeller or input shaft. The engine is selectively disconnected from the transmission using special “disconnect clutch”. The disconnect clutch allows vehicle to be driven under electric power alone, in hybrid mode with both electric machine and the engine propelling the vehicle, or in a combustion engine only mode where vehicle is propelled by the engine only.
Better fuel economy engine may be achieved by shutting down the engine when vehicle is decelerating and restarted when the driver depresses the accelerator pedal, or “tips in.” The engine may be disconnected from transmission and regenerative braking can be initiated when the brakes are applied to capture vehicle kinetic energy.
One problem with MHT vehicles is that the electric machine may not be able to provide the requested additional torque without the engine. For example, when the driver demands a large increase in torque (or in extreme cases the driver demands wide open throttle) in the middle of regenerative braking or when the vehicle was stopped with the engine shut down and disconnected, the engine has to be restarted quickly to provide adequate torque to meet the driver's demand for torque.
The engine is normally started by the electric machine with the disconnect clutch applied. Starting the engine with electric machine requires a certain portion of the electric machine torque to be used for the engine restart further slowing or delaying vehicle launch.
This disclosure is directed to solving the above problem and other problems associated with hybrid vehicles as summarized below.